1. Field of the Invention
This invention relates to an apparatus for removing acid constituents such as HCl and so-called SO.sub.x from the waste-gas generated in a combustion apparatus such as a combustion furnace, an incinerator and the like which burns materials containing elements of Cl and/or S, by the use of calcium-containing chemisorbent. It is intended by the term "chemisorb" to mean chemically absorb or adsorb, or absorb or adsorb with simultaneous chemical reaction.
2. Discussion of Background and Material Information
It has been known that the waste-gas generated by refuse incinerators includes a small amount of HCl gas originating from Cl-containing organic polymers such as polyvinyl chloride and polyvinylidene chloride. Again, waste-gas resulting from combustion of sulfur-containing materials is invariably contaminated by SO.sub.x or gaseous oxidized products of sulfur, with, in general, about 99 percent thereof being SO.sub.2 and the rest, SO.sub.3.
There are two types of gas purification processes of the wet-process and the dry-process. The former emcompasses scrubbing the gas with an aqueous solution or suspension of such a chemisorbent as NaOH, Na.sub.2 SO.sub.3, CH.sub.3 COONa, NH.sub.3, Ca(OH).sub.2, Mg(OH).sub.2, MgSO.sub.3, or basic aluminum sulfate. Whereas the wet-process has the merits or advantages of intensive removability, easy workability and scarceness of environmental pollution, in general the equipment required for this purpose is rather complex and expensive and requires a considerable amount of space for its construction. On the other hand, the dry-process is advantageous in that it is in smaller in size, less expensive to construct, and is simpler in operation.
The chemisorbants used in the dry-process are mostly calcium compounds mainly for economical reasons, although aluminum coated with sodium oxide, hydrated manganese oxide and copper oxide were once claimed to be effective. Of the calcium compounds, calcium hydroxide, calcium oxide and calcium carbonate are by far the most suitable in view of availability, effectiveness and economy. Among them, calcium carbonate is somewhat different from the others in that it is a salt and less reactive. However, the occurrence of the following chemical reactions at higher temperatures of about 800.degree. to 1000.degree. C. makes choice among them less important. EQU CaCO.sub.3 --CaO+CO.sub.2 ( 1) EQU Ca(OH).sub.2 --CaO+H.sub.2 O (2)
Care should be taken in the case of CaCO.sub.3, therefore, to maintain these temperatures for sufficient time to enable the reaction (1) to proceed.
In a typical conventional dry-process using calcium hydroxide as the chemisorbent, particulate calcium hydroxide suspended in air is blown into the duct of waste-gas maintained at higher temperatures, to form a lean fluidized flow within the duct which moves downstream. During its migration a substantial amount of chemisorption of the acid gases occurs, and eventually reaches an electrostatic precipitator wherein the solid powder is electrostatically removed. In the case where calcium carbonate is used as the chemisorbent, it is preferable to charge the powder into higher temperature zones of above 800.degree. C. to increase the rate of the aforementioned reaction (1).
The amount of the calcium compounds used was at most two times the stoichiometric or chemical equivalent because excess amounts incur squandering of electrical energy and are not economical. The extent of dechlorination is commonly 40 to 60%, the same being the case for desulfurization.
It will be desirable that the extent of removal of the acid constituents is increased without increased load for the electrostatic precipitator.